In the high precision machine tool industry, it is vital to rapidly machine all work pieces to tight dimensional tolerances with smooth surface finish on a repeatable basis. Of particular interest is production of large, relatively thin cross section, contoured and/or complex work pieces, accurately, quickly and repeatedly. These types of work pieces are very difficult to hold rigidly, accurately and inexpensively because they are inherently susceptible to chatter. Chatter is the high frequency vibration of the work piece surface against, for example, a moving or rotating cutting tool. Chatter causes individual height differences on each pass of the tool across the work piece surface. Whenever such differences in height occur, a sharp (90°) corner is created, at each of which is concentrated static, dynamic and cyclic tensile stresses.
Stress concentrations can initiate both stress corrosion cracking and corrosion-fatigue in the presence of an atmospheric corrodent. Stress corrosion cracking is a progressive fracture mechanism in metals which is caused by simultaneous interaction of a corrodent and sustained tensile stress during service. Corrosion-induced fatigue can also cause a somewhat similar fracture mechanism which causes progressive cracking due to cyclic stress loading during service. Either fracture mechanism can lead to catastrophic failure of a work piece in normal service.
Prior attempts to reduce work piece chatter include modifications to the work piece itself, as well as re-designing cutter tool geometries, machining parameters, and work piece setups. None provide vibration damping of thin cross-section work pieces or temperature control of the work piece. Other chatter reduction approaches include use of modeling clay, non-linear metallic springs, gas and hydraulic shock absorbers, tuned resonant structures and mechanical preloading. Attempts to use elastic materials rely on partial compression of a compliant material, resulting in a lack of rigidity in supporting the work piece. Since the work piece is not held rigidly, shallower depths of cut at slower tool rotation, smaller step-over and/or slower feed rate are required to mill cleanly without producing waves or sharp shoulders in the surface finish.
Compliant or visco-elastic materials have a very low thermal conductivity and act as heat insulators, rather than as heat sinks. Since the machining process creates heat energy, some of the heat energy does transfer away from the work piece as very hot chips are ejected from the work piece. However, at best, only 75% of the heat energy is exported in the chips. The remaining heat energy is absorbed by both the tool/spindle and the work piece. Whatever remaining heat energy that does not transfer to the tool/spindle builds up in the work piece. Also, compliant materials may set up an entirely new set of oscillation frequencies in the work piece. Thus, while they may damp some vibration frequencies, they can set up a different, typically lower, vibration frequency that has a higher amplitude motion in the work piece. Where visco-elastic material is applied along the outside marginal edges of the work piece, or an insufficient amount is used, other oscillations are set up that were not present when the work piece was held without their use. Experience shows that combinations of such chatter reduction methods both lengthen cycle times and concentrate heat in the work pieces. Due to the deficiencies of such prior anti-chatter work piece holding methods, significant quantities of reject work pieces are created, requiring costly, manual rework with poor results.
Both chatter and thermal build-up are exacerbated in “dry” machining where no coolant or lubricant is used. The aerospace industry dry machines work pieces because they are too large to machine “wet”. Most long aerospace work pieces are dry machined from aluminum because of the size of the part ranges in length from 50 to 112 feet long. Aluminum has high Thermal Conductivity and Thermal Coefficient of Expansion (TCE or CTE). A large, thin, contoured, asymmetrical work piece of aluminum is in real life a dimensional moving target during machining. Expansion and contraction rates can vary dramatically from one work piece to the next.
Recent precision specifications issued domestic and foreign aero-space companies have dictate that the thickness of these long, thin, contoured-surface work pieces must be made to extremely tight tolerances. These new specifications require that the thickness of each individual work piece be measured at many representative points to high accuracy tolerances of ±0.003″. These measurements and locations must be recorded in order to meet contractual requirements. This new specification presents a two fold problem: First, the machine tool itself must be much more accurate and repeatable. Second, TCE-driven thickness changes of the work piece must be controlled to stay within overall dimensional tolerance. At present, this new quality specification is unmet by original equipment manufacturing (OEM) vendors of structural frame components.
Even after the inspected work piece is hung vertically between operations, it warms up to the ambient temperature. If a 50′ work piece is machined at 50° F., when taken off of the machine bed, it warms up, e.g., to 68°±1° F., the total end-to-end TCE-driven expansion is approximately +0.140 inches. Any dimensional feature that was initially in the center of the ±0.030″ tolerance window but is located more than 150″ (12.5′) from the reference end will have expanded beyond the allowable distance from the end. The work piece that was acceptable when cool has thermally expanded into a low-quality non-sellable reject, and must be reworked or scrapped. These issues are exacerbated by the aircraft industry's adoption of higher strength, Alcoa 7075-B alloy, since chatter problems may make the most efficient machining of the high strength Al alloy slower, while machining, heat-induced TCE would make precision impossible.
Thus, there remains a severe, urgent, un-met need in the art to provide a solution to the serious and costly problems of dry cutter friction driving TCE-induced expansion of the work piece, and cutter/work piece interaction chatter that results in work-piece surface finish irregularities and off-specification dimensional errors. There is also a need for an adaptable system for a wide variety of machining operations on work pieces of complex shapes, large sizes and thin cross-sections, yet is simple to install and operate, is relatively inexpensive, can be retrofit on to previously installed large machine tools, and can be used to produce high quality dry-machined parts, in shorter run time and at greater yield, particularly in the aerospace industry.